This invention relates generally to apparatus and processes for aerating dispersions. More particularly, the present invention relates to apparatus and processes for aerating pulp suspensions during de-inking.
De-inking flotation is a mechanical process for removing impurities and ink particles from pulp suspensions produced particularly in waste paper treatment. This process requires the generating of gas bubbles in the appropriate quantity and size distribution. Hydrophobic substances or substances to which ampholytics are added to make them hydrophobic, such as ink particles or stickies, are carried to the surface of the liquid by the gas bubbles adhering to them and can be removed from the surface as scum. This is referred to as selective flotation because the pulp is discharged with the accept due to its hydrophile nature. Processes of this type are known in numerous geometric modifications, for example from DE 41 16 916 C2 or EP 0 211 834 B1, and have reached a high technical standard. Further, it has also proved successful to use self-priming injectors to generate gas bubbles and mix these with the pulp suspension. These injectors basically comprise a propulsive jet nozzle, a mixing or impulse exchange pipe, and a diffuser. Here, the liquid flow emerging from the propulsive jet nozzle according to the open jet principle generates under pressure. As a result, gas is sucked in and mixed with the liquid as a result of the impulse exchange between liquid and gas in the mixing pipe. At the exit from the diffuser used for energy recovery a dispersion of pulp and bubbles is formed. Use of the known processes and injectors, however, has shown several disadvantages in selective flotation of pulp suspensions.
The suction effect of the known injectors in operation with pulp suspensions is too weak and the bubble size distribution generated by the injector known does not have the optimum design to meet the requirements of selective flotation.